Monilethrix is an autosomal dominant disorder chiefly affecting hair. The degree of hair dystrophy is highly variable, as is the presence of additional features, such as follicular keratoses. In three British families of monilethrix, linkage has recently been reported to the type II keratin gene cluster at chromosome 12q13, and it has been suggested that the disease is due to a defect in the hard keratins of hair and nail. If monilethrix is a keratin disorder, we would predict that some pedigrees might map to the type I keratin gene cluster on 17q where hard keratin genes are also found. We have now studied clinically and by linkage analysis three new and unrelated pedigrees from England, Scotland and Spain, the first of which showed a variant phenotype. In this family the disease was expressed in four of 12 cases only as a follicular keratosis of the neck, elbows and knees, and without clinical or historical evidence of hair anomalies: non-penetrance in an obligate carrier was also observed. In all three families, we have established linkage to a series of microsatellite markers at the type II locus at 12q13 (Zmax=6.34 at 0=0.00 for D12S368) and have excluded linkage from the type I keratin gene cluster on 17q. It remains probable that monilethrix is a disorder of hard keratins, but at present there is no evidence that it is due to defects in type I keratins.